1. Field of the Invention
The present invention relates to a power-operated chuck for supporting cylindrically shaped workpieces which are to be machined by means of a machine tool in the area of their free ends.
2. Description of the Prior Art
Power-operated chucks of this kind have been produced successfully by the Applicant for decades under the identification Big Bore® and placed on the market. These power-operated chucks have a passage opening extending in the lengthways axis of the power-operated chuck, in order to accommodate a workpiece to be machined.
In order to position the cylindrically shaped workpiece as exactly centrally as possible in relation to the lengthways axis of the chuck, the chuck body of the chuck is provided with three centring jaws arranged at 120° intervals from one another in the circumferential direction of the workpiece, and which are advanced synchronously by means of a drive piston in the direction of the workpiece, with the effect that the centring jaws simultaneously make active contact with the workpiece, or change the position of the workpiece in such a way that, once the centring jaws have been completely advanced, the workpiece is aligned exactly centrally in relation to the lengthways axis of the power-operated chuck.
For this purpose, the three centring jaws are mounted in a guide groove in a manner which allows radial movement, because the axial advance movement of the drive element in the form of a piston rod is redirected by means of a wedge bar when the axial advance movement has finished. The result is an axial advance movement of the piston rod onto the wedge bar, in a manner which approaches as far as possible the area of the free end of the workpiece to be machined, so as to secure it in the space centrally in relation to the lengthways axis, since it is in this area that machining of the workpiece takes place. Following this, the centring jaws slip along the wedge bar in a radial direction, and enter into active contact with the workpiece.
Once the workpiece has been positioned centrally by means of the three centring jaws, three clamping jaws inserted in the chuck are advanced towards the workpiece, these clamping jaws being arranged between the centring jaws in the chuck. As soon as the clamping jaws enclose the positioned workpiece, the centring jaws can be separated from the workpiece in order to release it for machining. As long as the centring jaws continue to act on the workpiece, it is inaccessible for machining. As a result, the clamping jaws move through a plane that is at a distance from the plane formed by the centring jaws.
Although power-operated chucks of this kind have proven effective in practice, it has nevertheless proven to be a disadvantage that the centring jaws or the entire power-operated chuck must be replaced for differently sized diameters of workpiece, because the axial advance travel, as well as the radial movement of the centring jaws over the sloping plane of the wedge bar, have to be selected such that the centring jaws enter into active contact with the workpiece to be machined as close as possible to the free end of the workpiece. Such changeover measures take considerable time and are thus cost-intensive, since the machine tool is stopped during the period of the changeover.
A power-operated chuck with a modified design is disclosed in DE 10 2006 027 988 A1; this chuck has a total of three radially advancing clamping jaws that enter into active contact with the workpiece to be clamped, thereby centring it in the space. For the purpose of actuating the clamping jaws, they are connected to a driving means by an actuating pin. For this purpose, the chuck body is provided with a guide groove worked into it at an angle vectored towards the workpiece or the clamping jaws, in which guide groove the corresponding actuating pin is mounted in an axially movable arrangement.
DE 3304876 A1 also discloses a power-operated chuck of this kind.
It is a disadvantageous feature that the driving means requires a space outside the chuck, since this acts directly on the actuating pin due to the fact that the power transmission takes place onto the actuating pin by means of the driving means in an axial direction. In this case, the advance force is generated by means of a cylinder housing of the driving means, and acts along an axial piston rod. The cylinder housing has an opening arranged pointing radially outwards, which is used for accommodating a driver pin. However, a design of this kind increases the space required by the power-operated chuck considerably in the outwards direction. Often, however, power-operated chucks of this kind are installed in machine tools which have restricted space, as a result of which power-operated chucks of this size cannot be used.